Simultaneous enhancement of transmission loss and absorption coefficient using activated cavities

ABSTRACT

A method and apparatus for providing simultaneous enhancement of transmission loss and absorption coefficient using activated cavities is presented. A layer of material is provided, and a backing plate having a plurality of cavities on the top surface of said backing plate, is disposed adjacent a top surface of said layer of material. A screen is disposed along the top surface of said cavities on said backing plate and at least one cavity includes an actuator disposed within the cavity and a control system receiving a signal from the microphone and receiving a signal from the accelerometer and providing a drive signal to the actuator to provide an acoustic output to provide simultaneous insertion loss and absorption which serves to minimize a linear combination of the signal from the microphone and the signal from the accelerometer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 11/756,182 entitled “SIMULTANEOUS ENHANCEMENT OF TRANSMISSIONLOSS AND ABSORPTION COEFFICIENT USING ACTIVATED CAVITIES,” filed on May31, 2007 which is hereby incorporated herein by reference in itsentirety.

BACKGROUND

The control of interior noise is a formidable problem. The sources ofnoise are many and varied; for example, in modern turbo-fan commercialaircraft, the turbulent boundary layer (TBL), engine noise and auxiliaryequipment noise can all contribute to the interior noise. The primarysource of interior noise is the turbulent boundary layer that formsaround the exterior surface of the aircraft in flight, with theadditional contribution of noise from the engines towards the rear ofthe aircraft. The random pressures generated in the TBL cause motion ofthe aircraft sidewalls, which then radiate sound into the interior. Thissound then propagates through the walls and ceiling and into theinterior of the aircraft. Reflected sound from the interior surfacesbuilds up a semi-reverberant noise field.

Current conventional noise control treatments are effective at moderateto high frequencies and so the residual noise spectrum is dominated bylower frequency noise centered around 300 Hz. At these frequencies, theuse of conventional treatments is hampered by severe weight and volumeconstraints. Active noise control offers the potential of effectiveperformance without a significant weight penalty.

Several studies, using a variety of approaches, have considered theapplication of active noise control for aircraft interior noise. A fewsystems have been fully developed, are available commercially and are inregular service, specifically for the control of tonal noise in theinterior of propeller aircraft. Systems proposed for broadband interiornoise include control of fuselage vibration, activated absorptivetreatments, activated panels and enhanced double wall panel systems.Each of these systems has advantageous features and implementationdrawbacks.

SUMMARY

Conventional mechanisms such as those explained above suffer from avariety of deficiencies. One such deficiency is that there is no singlesolution that provides both broadband transmission loss through thecabin wall of an aircraft and improved absorption in the cabin interior.Sound absorption by materials is typically limited by their size;thicker materials are required to absorb lower frequencies. A classicsound absorber is a cavity faced by an absorbent screen. When the depthof the cavity is one quarter of the wavelength of incident sound, apressure null occurs inside the screen, reducing the local sound fieldand forcing the incident sound wave to expend energy by an increasedacoustic particle velocity through the resistance of the screen. The useof these devices in aircraft is limited by the wavelength of typicalnoise sources; to be effective at 1000 Hz, a quarter-wave cavity must beapproximately 4 inches deep.

Embodiments of the invention significantly overcome such deficienciesand provide mechanisms and techniques that provide noise control. Thepresently disclosed noise control system uses active components toenhance the performance of well-established passive noise controltreatments and provides both broadband transmission loss through thecabin wall and improved absorption in the cabin interior.

Active-passive cavity absorbers use a shallow passive cavity absorberwhose frequency range is extended by the inclusion of the combination ofa small actuator (e.g., a loudspeaker) and a microphone, coupled by alocal feedback loop which serve to reduce the sound pressure in thecavity at low frequencies. The effect of the active component is toextend the range of frequencies at which absorption occurs to much lowerfrequencies. The combination of the passive sound absorber enhanced byan active system, greatly reduces control interaction when multipledevices are used together, increasing the robustness of the combinedsystem. Again, the use of multiple single input, single output (SISO)local control systems significantly reduces system complexity andinstallation and maintenance costs. All control, signal conditioning andpower electronics are integral with the actuator unit; only a connectionfor electrical power is required.

Noise control is achieved by providing a layer of contiguous cavitieswith the backside closed and the front side covered with a flowresistive screen that forms a high transmission loss barrier. Withineach cavity is an actuator, such as a loudspeaker or the like. At thefront of each cavity is a microphone that measures the sound pressurenear the flow resistive screen and an accelerometer that measures theacceleration of the cavity in the direction normal to the flow resistivescreen. A control system drives received inputs from the microphone andaccelerometer and provides an output that drives the actuator, so thatan acoustic output of the actuator provides simultaneous insertion lossand absorption.

In a particular embodiment of a method for providing noise control, themethod begins with receiving a signal from a microphone disposed withina cavity. The method also includes receiving a signal from anaccelerometer disposed within the cavity. Additionally the methodincludes providing a drive signal to an actuator to produce an acousticoutput of the actuator that provides simultaneous insertion loss andabsorption, which serves to minimize a linear combination of the signalfrom the microphone and the signal from the accelerometer. It is to beunderstood that the embodiments of the invention can be embodied assoftware and hardware, or as hardware and/or circuitry alone.

In a particular embodiment within each cavity is an actuator. While theactuator could be realized as a speaker, in another embodiment theactuator comprises a first shell, a second shell and a driver (e.g., apiezoelectric patch) disposed on the first shell. The first shell andthe second shell enclose a volume of air at a reduced pressure. Eachcavity further includes two sensors, a microphone and an accelerometer,and a control system. The control system receives a signal from themicrophone and receives a signal from the accelerometer and provides adrive signal to the actuator to produce an acoustic output from theactuator to minimize a linear combination of the signal from saidmicrophone and the signal from said accelerometer. The sensors and theactuator are components of an active system that enhance both theinsertion loss and the absorption, decreasing the noise that is bothtransmitted through the cabin wall and incident on the wall from othersources (or due to reverberant build up) in the cabin.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 depicts a top view of a particular embodiment of a noise controlsystem in accordance with embodiments of the invention;

FIG. 2 depicts a cross-sectional view of a particular embodiment of anoise control system in accordance with embodiments of the invention;

FIG. 3 depicts a schematic diagram of a particular embodiment of a noisecontrol system in accordance with embodiments of the invention showingdifferent sound waves;

FIG. 4 depicts a block diagram of a first controller;

FIG. 5 depicts a block diagram of a second controller;

FIG. 6 comprises a flow diagram of a particular embodiment of a methodfor providing noise control in accordance with embodiments of theinvention;

FIG. 7 comprises a flow diagram of a particular embodiment of a methodof providing an actuator in accordance with embodiments of theinvention; and

FIG. 8 comprises a flow diagram of a particular embodiment of a methodfor reduction of radiated sound in accordance with embodiments of theinvention.

DETAILED DESCRIPTION

Embodiments of the noise control system of the present inventionincorporate both passive and active components that work in synergy toprovide broadband transmission loss through the cabin wall of anaircraft and improved absorption in the cabin interior. An exampleembodiment 10 is illustrated in FIGS. 1 and 2. A layer of material 20(e.g., a low density, micro-fiber glass fiber mat) is attached, or inclose proximity, to the fuselage wall 26 on the interior of the aircraftor to the inner wall of the cabin of a vehicle. A layer of smallcavities 18 is disposed above the glass fiber mat 20, such that a smallair gap is present between the cavities 18 and the glass fiber mat 20.The cavities are defined by a backing plate 28 into which in oneembodiment are inserted actuators composed of elements 14, 16, 32 and30. A screen 12 is disposed along the top surface of the cavities 18.The first role of the cavities 18 is to provide mass loading that willenhance the passive insertion loss of the glass fiber mat 20. Inaddition, each cavity 18, with its flow resistive front face, acts as anacoustic absorber for noise incident from the cabin interior.

Within each cavity 18 is an actuator. While the actuator could berealized as a speaker, in another embodiment the actuator comprises afirst shell 16, a second shell 30 and a driver 14 (e.g., a piezoelectricpatch) disposed on the first shell 16. The first shell 16 and the secondshell 30 enclose a volume of air at a reduced pressure 32. Each cavity18 further includes two sensors, a microphone 22 and an accelerometer24, and a control system. The control system receives a signal from themicrophone and receives a signal from the accelerometer and provides adrive signal to the actuator to produce an acoustic output from theactuator to minimize a linear combination of the signal from saidmicrophone and the signal from said accelerometer. The sensors 22 and 24and the actuator are components of an active system that enhance boththe insertion loss and the absorption, decreasing the noise that is bothtransmitted through the cabin wall and incident on the wall from othersources (or due to reverberant build up) in the cabin. While thecavities may be of any size, in a particular embodiment the cavitiesmeasure a few inches across and approximately one inch deep.

The active system controls a weighted sum of the inputs from each sensorwhere the weighting is selected to ensure that both noise control goalsare met simultaneously. The actuator is both lightweight and capable ofgenerating sufficient acoustic output at low frequencies.

In the noise control system one passive mechanism for transmission lossis the mass glass fiber mat 20. Sound impinging on the outside surfaceof the fuselage, or cabin wall, causes vibration of the wall 26. This,in turn, excites the layer of glass fiber mat 20. At low frequency themass provided by the layer of thin activated cavities interacts with thestiffness of the glass fiber to provide an impedance discontinuity,which attenuates the transmission of sound into the aircraft interior.At higher frequency the mass of the layer of thin cavities on its ownprovides the impedance discontinuity that enhances the transmission lossof the fuselage or cabin wall. Careful choice of the properties of theglass fiber can enhance attenuation at lower frequencies over what couldbe achieved with the mass of the layer of cavities alone.

Another passive mechanism for sound absorption is the cavity absorber.At certain frequencies, typically at the quarter wave resonances of thecavity depth, incident sound has a high acoustic particle velocity atthe entrance to the cavity. The flow resistive screen 12 at the entranceto the cavity resists this particle velocity and absorbs the energy ofthe incident sound. This principle is used in resonant Helmholtzabsorbers in industrial noise control applications and in the acousticliners of turbofan engines. As with the passive insertion loss noisecontrol, this mechanism is most effective at higher frequencies.However, with activation, as described to below, the effectiveness canbe extended to low frequencies

The goal of the active control system is two fold. The active controlsystem increases the insertion loss by generating sound that interfereswith the sound radiated by the cavities due to their motion normal tothe flow resistive layer. The active control system also extends thefrequency range of the absorber by actively creating a low acousticpressure in the cavity, mimicking the conditions created by a quarterwave resonance but across a wide band of frequencies. In order toachieve both these goals simultaneously two sensors are used, amicrophone 22 to detect the sound pressure in the cavity and anaccelerometer 24 to detect the vibration of the radiating face of thecavity.

The actuator is capable of producing sufficient low frequency sound yetis both small and lightweight. While in one embodiment the actuator isrealized as a loudspeaker, in another embodiment, the actuator design isbased on the concept of a vacuum bubble. Vacuum bubbles were originallydeveloped as a noise control solution to the problem of implementing alayer of low impedance in air. High transmission loss can be obtained byimplementing an impedance discontinuity. In air, a high impedancebarrier requires mass. A low impedance barrier though lightweight ismore of a challenge, particularly in air, as very few materials have alower characteristic acoustic impedance than air.

A vacuum bubble is a metal shell with a very thin skin enclosing avolume in which the internal air pressure has been reduced. The shell isdesigned so that an in-plane compressive stress develops as the internalpressure is reduced. The in-plane stress reduces the effective bendingstiffness of the shell. The reduced shell stiffness, together with thereduced stiffness of the internal air volume, results in a verycompliant volume.

The actuator actively drives a vacuum bubble shell using one or morepiezoelectric patch drivers. The internal volume is formed by two veryshallow sections of spherical shell attached to a light backing platethrough which a hole (not shown) has been stamped. The thin shells oneach side of the hole in that backing plate form an airtight volume. Inone embodiment, two shells are each equipped with a piezoelectricceramic patch rather than one to avoid inducing bending stresses andinertial reaction forces on the backing plate. When the pressure in theinternal volume is reduced, compressive stresses are induced in theshells and reaction tensile stresses in the backing plate. Apiezoelectric ceramic patch is bonded to the surface of each shell, suchthat when activated the shell deforms increasing and decreasing thevolume. By reducing the pressure in the internal volume, the complianceof the volume and shell can be significantly increased, lowering theactuator natural frequency and improving actuator output.

The strategy and mechanism of this control approach can be illustratedby considering a simplified one-dimensional system, shown in FIG. 3. Inthis Figure a sound wave 40 is radiating from the front face of thecavity into the cabin. Part of this radiating wave is due to reflectionof an incident wave 42 from noise sources in the cabin, part is due tomotion of the face of the cavity due to vibration of the cabin wall(through the glass fiber mat which is here neglected). There is also apair of sound waves in the cavity, 44 and 46 which are shown forcompleteness but are not cogent to the discussion to follow. The passageof sound through the flow resistive screen causes a pressure dropproportional to the local acoustic particle velocity. The actuator isdriven so that the resulting sound pressure at the microphone 22 is p.The sound radiating from the face of the cavity into the cabin, p_(r),is a combination of reflected incident sound p_(i) and radiation due tothe forced motion caused by wall vibration u. If the flow resistance ofthe screen is z₁ and the characteristic impedance of the air is z₀ thenit can be shown that for one dimensional propagation the radiated soundpressure p_(r), is given by

$p_{r} = {{\frac{1 - a}{1 + a}p_{i}} + {\frac{a}{1 + a}( {p + {z_{1}u}} )}}$where p_(r) is the radiated sound pressure, a is the impedance ratio

$\frac{z_{0}}{z_{1}},$p_(i) is the incident sound, z₁ is the flow resistance of the screen andu is the wall vibration.

The component of radiated sound due to reflection can be eliminated byselecting a=1, that is by selecting the flow resistance of the screen tobe that of the characteristic impedance of the air. The component of theradiated sound due to motion of the cavity can be eliminated by drivingthe summed signal e=p+z₁u to be minimal. It is interesting to note thatthe control system removes the impedance of the cavity behind the screeneffectively extending its quarter wave resonance to low frequency. Thisis accomplished not by driving the pressure in the cavity to zero, butby regulating the pressure to be approximately −z₁u.

A practical installation of the noise control system has many smallcavities each with its local sensors and actuators. The objective of theactive control system is to drive each actuator with the signal requiredto achieve the acoustic condition of simultaneous insertion loss andabsorption. In general, the active control system comprises a multipleinput multiple output (MIMO) control system design problem in that theoutput of each actuator can, to some degree, cause an output at eachsensor in the array of cavities. The hybrid nature of the present noisecontrol system that has passive noise control elements as itsfoundation, will tend to minimize any cavity to cavity interactionpotentially leading to the goal of noise control that can be achieved byusing individual local control systems in each cavity. Although eachcontrol system may be local in that its output actuator drive signaldepends only on the inputs from the local microphone and accelerometer,the design of the local control function may make use of information ofcavity to cavity interaction to ensure performance and stable operation.

There are at least two options for the robust design of the controllertransfer function, including the Internal Model Control Method,illustrated in FIG. 4 and the Compensator-Regulator method, illustratedin FIG. 5. In the Inner Model Method controller 50 of FIG. 4, thecontrol filter in the main feedback loop has a secondary feedback looparound it as well. In that loop is a digital filter representation ofthe plant transfer function (the dynamic system being controlled). Thepresence of the plant transfer function in the feedback loop around thecontrol filter transforms the feedback controller into a feed forwardcontroller allowing all of the tools available for feed forward controlfilter design and adaptation to be applied to the feedback problem,including the filtered-X algorithm. To the extent that the digitalfilter representation of the plant transfer function is accurate, theInner Model architecture ensures that the feedback system will bestable.

In the Compensator Regulator Method controller of FIG. 5, the controlfilter is divided into two parts: a compensation filter and a regulationfilter. The purpose of the compensation filter is to remove the dynamicsof the plant from the feedback loop. As such, the compensation filter isconfigured to approximate the inverse of the plant transfer functionsuch that the product of the compensation filter and plant transferfunction is approximately unity. The regulation filter then provides thehigh gain in the feedback loop over the range of frequencies where goodperformance is desired.

One of the most important considerations in the design process will bethe true nature of the various physical transfer functions and theirvariability both over time and between units. The passive noise controlcomponents should be of benefit here as it is not expected to see themany lightly damped modes that are the bane of the designers of manyactive noise control system; the passive components will provide dampingto the plant transfer functions and will smooth the variability betweenunits. If a fixed SISO control structure is insufficient, we have theoption of extending the control strategy to include adaptive control,multiple input single output (MISO) control and banded multiple inputmultiple output (MIMO) control.

Flow charts of particular embodiments of the presently disclosed methodsare depicted in FIGS. 6-8. The rectangular elements are herein denoted“processing blocks” and represent computer software instructions orgroups of instructions. Alternatively, the processing blocks representsteps performed by functionally equivalent circuits such as a digitalsignal processor circuit or an application specific integrated circuit(ASIC). The flow diagrams do not depict the syntax of any particularprogramming language. Rather, the flow diagrams illustrate thefunctional information one of ordinary skill in the art requires tofabricate circuits or to generate computer software to perform theprocessing required in accordance with the present invention. It shouldbe noted that many routine program elements, such as initialization ofloops and variables and the use of temporary variables are not shown. Itwill be appreciated by those of ordinary skill in the art that unlessotherwise indicated herein, the particular sequence of steps describedis illustrative only and can be varied without departing from the spiritof the invention. Thus, unless otherwise stated the steps describedbelow are unordered meaning that, when possible, the steps can beperformed in any convenient or desirable order.

Referring now to FIG. 6, a particular embodiment of a method 100 ofproviding noise control is shown. The method 100 begins with processingblock 102 which recites receiving a signal from a microphone disposedwithin a cavity. The microphone measures pressure within the cavity andprovides an output signal which is used as part of a feedback loop inorder to provide an output signal for driving an actuator.

Processing block 104 discloses receiving a signal from an accelerometerdisposed within the cavity. The accelerometer measures the velocity ofthe cavity and provides an output signal which is used as part of afeedback loop in order to provide an output signal for driving theactuator.

Processing block 106 states providing a drive signal to an actuator toprovide an acoustic output to provide simultaneous insertion loss andabsorption. The provided signal serves to minimize a linear combinationof the signal from the microphone and from the signal from theaccelerometer. The actuator, in certain embodiments, may be realized asa speaker.

Processing continues with processing block 108 which recites whereinreceiving a signal from the microphone, receiving a signal from theaccelerometer, and providing a drive signal to the actuator to providean acoustic output to provide simultaneous insertion loss andabsorption, which serves to minimize a linear combination of the signalfrom the microphone and the signal from the accelerometer, is performedby a control system and wherein the control system has a controllerarchitecture selected from the group consisting of an internal modelcontrol and a compensator-regulator.

In the Inner Model Method controller, the control filter in the mainfeedback loop has a secondary feedback loop around it as well. In thatloop is a digital filter representation of the plant transfer function.The presence of the plant transfer function in the feedback loop aroundthe control filter transforms the feedback controller into a feedforward controller allowing all of the tools available for feed forwardcontrol filter design and adaptation to be applied to the feedbackproblem, including the filtered-X algorithm. To the extent that thedigital filter representation of the plant transfer function isaccurate, the Inner Model architecture ensures that the feedback systemwill be stable.

In the Compensator-Regulator Method controller, the control filter isdivided into two parts: a compensation filter and a regulation filter.The purpose of the compensation filter is to remove the dynamics of theplant from the feedback loop. As such, the compensation filter isconfigured to approximate the inverse of the plant transfer functionsuch that the product of the compensation filter and plant transferfunction is approximately unity. The regulation filter then provides thehigh gain in the feedback loop over the range of frequencies where goodperformance is desired. All these serve to provide both broadbandtransmission loss and improved absorption.

Referring now to FIG. 7, a method 150 of providing an actuator inaccordance with embodiments of the invention is shown. The method beginswith processing block 152 which discloses providing a first shelldisposed within the cavity and wherein a first driver is disposed on thefirst shell. Preferably, the first shell comprises a metal shell with avery thin skin which (in combination with a second shell) encloses avolume in which the internal air pressure has been reduced. The shell isdesigned so that an in-plane compressive stress develops as the internalpressure is reduced. The in-plane stress reduces the effective bendingstiffness of the shell. The reduced shell stiffness, together with thereduced stiffness of the internal air volume, results in a verycompliant volume. The driver may be realized as a piezoelectric ceramicpatch which is bonded to the surface of the shell, such that whenactivated the shell deforms increasing and decreasing the volume. Byreducing the pressure in the internal volume, the compliance of thevolume and shell can be significantly increased, lowering the actuatornatural frequency and improving actuator output.

Processing continues with processing block 154 which states providing asecond shell abutting the first shell such that the first shell and thesecond shell enclose a volume of air at a reduced pressure therein. Thesecond shell also comprises a metal shell with a very thin skin which(in combination with the first shell) encloses a volume in which theinternal air pressure has been reduced.

Processing block 156 recites providing a drive signal to an actuatordisposed within a cavity to produce an acoustic output which providessimultaneous insertion loss and absorption to minimize a linearcombination of the signal from the microphone and the signal from theaccelerometer. As shown in processing block 158 providing a drive signalto an actuator disposed within a cavity to produce an acoustic outputwhich provides simultaneous insertion loss and absorption to minimize alinear combination of the signal from the microphone and the signal fromthe accelerometer is provided by providing the drive signal to the firstdriver.

Processing block 160 discloses providing a second driver disposed on thesecond shell and wherein the second driver and the second shell producea drive signal to the actuator disposed within a cavity to produce anacoustic output which provides simultaneous insertion loss andabsorption to minimize a linear combination of the signal from themicrophone and the signal from the accelerometer in conjunction with thefirst driver and the first shell.

Referring now to FIG. 8, a particular embodiment of a method 200 foreliminating radiated sound is shown. The method 200 begins withprocessing block 204 which recites eliminating radiated sound due tomotion of the at least one cavity by driving a summed signal to aminimal value, the summed signal comprising the sound pressure at themicrophone added to a low resistance of the screen combined withradiation caused by vibration of a wall of the cavity. One manner toachieve this is shown in processing block 206 which discloseseliminating radiated sound due to motion of the at least one cavity bydriving a summed signal to a minimal value, the summed signal edetermined in accordance with the formula e=p+z₁u wherein e is thesummed signal, p is the sound pressure at the microphone, z₁ is the flowresistance of the screen, and u is the velocity of the face of thecavity.

A noise control system and method utilizes active components to enhancethe performance of well-established passive noise control treatments andprovides both broadband transmission loss through the cabin wall andimproved absorption in the cabin interior. Noise control is achieved byproviding a layer of contiguous cavities with the backside closed andthe front side covered with a flow resistive layer that forms a hightransmission loss barrier. Within each cavity is an actuator, such as aloudspeaker or the like. At the front of each cavity is a microphonethat measures the sound pressure near the flow resistive screen and anaccelerometer that measures the acceleration of the cavity. A controlsystem receives inputs from the microphone and accelerometer andprovides an output that drives the actuator, so that an acoustic outputof the actuator provides simultaneous insertion loss and absorption.

In a particular embodiment the actuator comprises a first shell, asecond shell and a driver 14 (e.g., a piezoelectric patch) disposed onthe first shell. The first shell and the second shell enclose a volumeof air at a reduced pressure. The control system receives a signal fromthe microphone and receives a signal from the accelerometer and providesa drive signal to the actuator to produce an acoustic output from theactuator to minimize a linear combination of the signal from saidmicrophone and the signal from said accelerometer.

Having described preferred embodiments of the invention it will nowbecome apparent to those of ordinary skill in the art that otherembodiments incorporating these concepts may be used. Additionally, thesoftware included as part of the invention may be embodied in a computerprogram product that includes a computer useable medium. For example,such a computer usable medium can include a readable memory device, suchas a hard drive device, a CD-ROM, a DVD-ROM, or a computer diskette,having computer readable program code segments stored thereon. Thecomputer readable medium can also include a communications link, eitheroptical, wired, or wireless, having program code segments carriedthereon as digital or analog signals. Accordingly, it is submitted thatthat the invention should not be limited to the described embodimentsbut rather should be limited only by the spirit and scope of theappended claims.

What is claimed is:
 1. A method of providing noise control comprising:receiving a signal from a microphone disposed within a cavity; receivinga signal from an accelerometer disposed within said cavity; providing adrive signal to an actuator to provide an acoustic output to providesimultaneous insertion loss and absorption which serves to minimize alinear combination of said signal from said microphone and said signalfrom said accelerometer; providing a first shell disposed within saidcavity and wherein a first driver is disposed on said first shell;providing a second shell abutting said first shell such that said firstshell and said second shell enclose a volume of air at a reducedpressure therein; and wherein said providing a drive signal to theactuator to provide an acoustic output to provide simultaneous insertionloss and absorption which serves to minimize a linear combination of thesignal from said microphone and the signal from said accelerometer isprovided by providing said drive signal to said first driver.
 2. Themethod of claim 1 further comprising providing a second driver disposedon said second shell and wherein said second driver and said secondshell provide an acoustic output to provide simultaneous insertion lossand absorption which serves to minimize a linear combination of thesignal from the microphone and the signal from the accelerometer inconjunction with said first driver and said first shell.
 3. The methodof claim 1 further comprising eliminating radiated sound due to motionof the cavity having a screen by driving a summed signal to a minimalvalue, said summed signal comprising the sound pressure at themicrophone added to a flow resistance of said screen combined with thevelocity of vibration of said cavity in the direction normal to the flowresistive screen.
 4. The method of claim 1 wherein said receiving signalfrom said microphone, said receiving a signal from said accelerometerand said providing a drive signal to said actuator to provide anacoustic output to provide simultaneous insertion loss and absorptionwhich serves to minimize a linear combination of the signal from themicrophone and the signal from the accelerometer is performed by acontrol system and wherein said control system has a controllerarchitecture selected from the group consisting of an internal modelcontrol and a compensator regulator.
 5. A method of providing noisecontrol comprising: receiving a signal from a microphone disposed withina cavity; receiving a signal from an accelerometer disposed within saidcavity; providing a drive signal to an actuator to provide an acousticoutput to provide simultaneous insertion loss and absorption whichserves to minimize a linear combination of said signal from saidmicrophone and said signal from said accelerometer; and eliminatingradiated sound due to motion of the cavity having a screen by driving asummed signal to a minimal value, the summed signal e determined inaccordance with the formulae=p+z ₁ u wherein e is the summed signal, p is the sound pressure at themicrophone, z₁ is the flow resistance of the screen, and u is thevelocity of vibration of said cavity in the direction normal to the flowresistive screen.